In radio base stations and other systems, power amplifiers are often used to amplify wideband signals or signal combinations with high peak to average power ratio (PAR or PAPR). The amplifiers must then be able to repeatedly output very high power for very short periods, even though the bulk of the output power is generated at the much lower average power level. In systems with random phase combinations of many signals (without any dominating ones) the amplitude of the signal follows a Rayleigh distribution.
A conventional single-transistor power amplifier (for example a class B, AB or F power amplifier) has a fixed radio frequency (RF) load resistance and a fixed voltage supply. The bias in class B or AB amplifiers causes the output current to have a form close to that of a pulse train of half wave rectified sinusoid current pulses. The direct current (DC) current (and hence DC power) is therefore largely proportional to the RF output current amplitude (and voltage). The output power, however, is proportional to the RF output current squared. The efficiency, i.e. output power divided by DC power, is therefore also proportional to the output amplitude. While efficiency is high at the highest output powers, the average efficiency of a power amplifier is consequently low when amplifying signals that on average have a low output amplitude (or power) compared to the maximum required output amplitude (or power), i.e. high PAR.
A Chireix amplifier (as described in “High power outphasing modulation,” Proc. IRE, vol. 23, no. 11, pp. 1370-1392, November 1935, by H Chireix), or a Doherty amplifier (as described in “A new high efficiency power amplifier for modulated waves”, by W. H. Doherty, Proc. IRE, vol. 24, no. 9, pp. 1163-1182, September 1936) were the first examples of amplifiers based on multiple transistors with passive output network interaction and combination.
They have high average efficiency for amplitude-modulated signals with high peak-to-average ratio (PAR) since they have a much lower average sum of RF output current magnitudes from the transistors at low amplitudes. This causes high average efficiency since the DC currents drawn by the transistors are largely proportional to the RF current magnitudes.
The reduced average output current is obtained by using two transistors that influence each other's output voltages and currents through a reactive output network (that is also coupled to the load). By driving the constituent transistors with the appropriate amplitudes and phases, the sum of RF output currents is reduced at all levels except the maximum. Also for these amplifiers the RF voltage at one or both transistor outputs is increased.
In 2001 the author of the present application invented two-stage high efficiency amplifiers with increased robustness against circuit variations and with radically increased bandwidth of high efficiency, as disclosed in patent number WO2003/061115 by the present Applicant. A wideband amplifier (100% relative bandwidth, i.e. having a 3:1 high band edge to low band edge ratio) has been successfully implemented by the present Applicant. The central mode of such an amplifier is a wideband Doherty mode.
By designing similar networks with more amplifiers and with transmission line networks with longer maximum electrical length, even wider bandwidths can be achieved, as shown for example in co-pending patent application number PCT/SE2013/051217. These amplifiers have a large total bandwidth of high efficiency even with small numbers of sub-amplifiers, for example even with three or four sub-amplifiers.
Wideband Doherty amplifiers are a subject of much interest, and many approaches have been attempted. For example, using a quarter wavelength transmission line with the same impedance as the load results in wideband efficiency at the transition point, as disclosed in a paper by D Gustafsson et al., entitled “Theory and design of a novel wideband and reconfigurable high average efficiency amplifier, Proc. IMS 2012.
The wideband multistage amplifiers of WO2003/061115 or PCT/SE2013/051217 have different operating modes in different frequency bands, which has the disadvantage of complicating the input drive circuits. The central Doherty mode of WO2003/061115 can be up to about 60% wideband, but the transition point amplitude then varies considerably within the bandwidth.
A Doherty amplifier that has a quarter wavelength line with the same impedance as the load, for example as disclosed in the paper mentioned above by Gustafsson, has the disadvantage of requiring a different supply voltage to each of the two sub-amplifiers. This results in an oversized and underutilized main transistor in case the same technology is used for both sub-amplifiers. The wideband efficiency at the transition point is obtained by sacrificing both wideband transistor utilization and efficiency at maximum power, which reduces the bandwidth of high average efficiency as well as increases transistor cost.
Using an LC-resonator, for example as disclosed in a paper by M Naseri Ali Abadi et al., entitled “An Extended Bandwidth Doherty Power Amplifier using a Novel Output Combiner”, Proc. IMS 2014, or using a resonant stub at the output node has the drawback of decreasing the full power bandwidth and efficiency bandwidth at full power (as opposed to the technique of WO2003/061115 that does not have this drawback).
Furthermore, using another technique involving the use of a multi-section branch line coupler has limitations in the efficiency bandwidth both at the transition point and at full power, and also power bandwidth at full power, at least in its present realizations as disclosed in a paper by Piazzon et al., entitled “A method for Designing Broadband Doherty Power Amplifiers”, Progress in Electromagnetics Research, Vol. 145, pp 319-331, 2014, or in a paper by R Giofré et al., entitled “A Distributed Matching/Combining Network Suitable to Design Doherty Power Amplifiers Covering More Than an Octave Bandwidth”, Proc. IMS 2014 (Based on abstract).